1. Field of the Invention
The present invention relates to an information recording medium, such as an optical disc, and an apparatus for and a method of reproducing information from the information recording medium.
2. Description of the Related Art
In an optical disc represented by a Compact Disc (CD) and a DVD, the long or short length of a pit is used to record main data. However, in order to record sub data including information for copy control for the prevention of illegal copying or the like, there is a request for reserving another recording area without decreasing a recording capacity by the pit.
As a method of increasing the recording capacity by other means except the method using the long or short length of the pit, there is known a technique of displacing or shifting the position of the pit in the radial direction of the optical disc. This technique is such that information is recorded by wobbling the position of the pit in the radial direction of the optical disc and by performing spread spectrum with respect to the wobble (e.g. Japanese Patent Application Laying Open NO. 2003-85896).
This technique is such as to use predetermined data as the sub data to append a synchronization signal to the predetermined data, perform spread spectrum with respect to the predetermined data by using random data to thereby generate a wobble signal, and wobble the position of a record mark according to the wobble signal. Since the wobble is spread in spectrum, it is difficult to detect the synchronization signal appended to the predetermined data. As a result, it is possible to increase the concealment of the predetermined data to some extent.
The main data has an appended synchronization signal for the main data corresponding to the synchronization signal of the predetermined data which is the sub data. The predetermined data is recorded onto the optical disc, by coinciding the start position of the predetermined data, which is indicated by the synchronization signal appended to the predetermined data, with the start position of the main data, which is indicated by the synchronization signal appended to the main data, and by wobbling the position of the record mark according to the wobble signal in the same area as an area into which the main data is recorded.
Thus, there is the possibility that the start position of the predetermined data is specified by reproducing the main data as well as the wobble signal from the optical disc and by detecting the synchronization signal appended to the predetermined data from the reproduced wobble signal on the basis of the synchronization signal appended to the reproduced main data. If the start position of the predetermined data is specified, the predetermined data is reproduced, which causes the problem that it is impossible to ensure the concealment of the information for copy control associated with copyright information or the like.